The 2013 Colorado Front Range Flood

Why was the Flooding so Destructive?

The St. Vrain River in Boulder County, CO after (left) and before
(right) the 2013 flooding event. Source: Boulder County via KRCC.

A major disturbance event like this flood causes significant changes in a
landscape. The St. Vrain River in the image above completely shifted its course
of flow in less than 5 days! This brings major changes for aquatic organisms,
like crayfish, that lived along the old stream bed that is now bare and dry, or
for, terrestrial organisms, like a field vole, that used to have a burrow under
what is now the St. Vrain River. Likewise, the people living in the house that
is now on the west side of the river instead of the eastern bank have a
completely different yard and driveway!

Why might this storm have caused so much flooding?

What other weather patterns could have contributed to pronounced flooding?

Introduction to Disturbance Events

Definition: In ecology, a disturbance event
is a temporary change in environmental conditions that causes a pronounced
change in the ecosystem. Common disturbance events include floods, fires,
earthquakes, and tsunamis.

Within ecology, disturbance events are those events which cause dramatic change
in an ecosystem through a temporary, often rapid, change in environmental
conditions. Although the disturbance events themselves can be of short duration,
the ecological effects last decades, if not longer.

Common examples of natural ecological disturbances include hurricanes, fires,
floods, earthquakes and windstorms.

Common natural ecological disturbances.

Disturbance events can also be human caused: clear cuts when logging, fires to
clear forests for cattle grazing or the building of new housing developments
are all common disturbances.

Common human-caused ecological disturbances.

Ecological communities are often more resilient to some types of disturbance than
others. Some communities are even dependent on cyclical disturbance events.
Lodgepole pine (Pinuscontorta) forests in the Western US are dependent on
frequent stand-replacing fires to release seeds and spur the growth of young
trees.

However, in discussions of ecological disturbance events we think about events
that disrupt the status of the ecosystem and change the structure of the
landscape.

In this lesson we will dig into the causes and disturbances caused during a storm
in September 2013 along the Colorado Front Range.

Driver: Climatic & Atmospheric Patterns

Drought

How do we measure drought?

Definition: The Palmer Drought Severity
Index is a measure of soil moisture content. It is calculated from soil
available water content,precipitation and temperature data. The values range
from extreme drought (values <-4.0) through near normal (-.49 to .49)
to extremely moist (>4.0).

Questions

In this dataset, what years are near normal, extreme drought, and
extreme wet on the Palmer Drought Severity Index?

What are the patterns of drought within Colorado that you observe using this
Palmer Drought Severity Index?

What were the drought conditions immediately before the September 2013
floods?

Over this decade and a half, there have been several cycles of dry and wet
periods. The 2013 flooding occurred right at the end of a severe drought.

There is a connection between the dry soils during a drought and the potential
for flooding. In a severe drought the top layers of the soil dry out. Organic
matter, spongy and absorbent in moist topsoil, may desiccate and be transported
by the winds to other areas. Some soil types, like clay, can dry to a
near-impermeable brick causing water to flow across the top instead of sinking
into the soils.

Atmospheric Conditions

In early September 2013, a slow moving cold front moved through Colorado
intersecting with a warm, humid front. The clash between the cold and warm
fronts yielded heavy rain and devastating flooding across the Front Range in
Colorado.

The storm that caused the 2013 Colorado flooding was kept in a confined area
over the Eastern Range of the Rocky Mountains in Colorado by these water vapor
systems.

Driver: Precipitation

How do we measure precipitation?

Definition: Precipitation is the moisture that
falls from clouds including rain, hail and snow.

Precipitation can be measured by different types of gauges; some must be
manually read and emptied, others automatically record the amount of
precipitation. If the precipitation is in a frozen form (snow, hail, freezing rain)
the contents of the gauge must be melted to get the water equivalency for
measurement. Rainfall is generally reported as the total amount of rain
(millimeters, centimeters, or inches) over a given per period of time.

Boulder, Colorado lays on the eastern edge of the Rocky Mountains where they meet
the high plains. The average annual precipitation is near 20". However, the
precipitation comes in many forms -- winter snow, intense summer thunderstorms,
and intermittent storms throughout the year.

The figure below show the total precipitation each month from 1948 to 2013 for
the National Weather Service's COOP site Boulder 2 (Station ID:050843) that is
centrally located in Boulder, CO.

Notice the general pattern of rainfall across the 65 years.

How much rain generally falls within one month?

Is there a strong annual or seasonal pattern? (Remember, with
interactive Plotly plots you can zoom in on the data)

Do any other events over the last 65 years equal the September 2013 event?

Now that we've looked at 65 years of data from Boulder, CO. Let's focus more
specifically on the September 2013 event. The plot below shows daily
precipitation between August 15 - October 15, 2013.

Driver: Stream Discharge

The USGS has a distributed network of aquatic sensors located in streams across
the United States. This network monitors a suit of variables that are important
to stream morphology and health. One of the metrics that this sensor network
monitors is stream discharge, a metric which quantifies the volume of water
moving down a stream. Discharge is an ideal metric to quantify flow, which
increases significantly during a flood event.

How is stream discharge measured?

Most USGS streamgages operate by measuring the elevation of the water in the
river or stream and then converting the water elevation (called 'stage') to a
streamflow ('discharge') by using a curve that relates the elevation to a set
of actual discharge measurements. This is done because currently the
technology is not available to measure the flow of the water accurately
enough directly. From the
USGS National Streamflow Information Program

What was the stream discharge prior to and during the flood events?

The data for the stream gauge along Boulder Creek 5 miles downstream of downtown
Boulder is reported in daily averages. Take a look at the interactive plot
below to see how patterns of discharge seen in these data?

And it isn't just floods, major hurricanes are forecast to strike New Orleans,
Louisiana once every
20 years.
Yet in 2005 New Orleans was pummeled by 4 hurricanes and 1
tropical storm. Hurricane Cindy in July 2013 caused the worst black out in New
Orleans for 40 years. Eight weeks later Hurricane Katrina came ashore over New
Orleans, changed the landscape of the city and became the costliest natural
disaster to date in the United States. It was frequently called a 100-year
storm.

If we say the return period is 20 years then how did 4 hurricanes strike New
Orleans in 1 year?

The return period of extreme events is also referred to as recurrenceinterval. It is an estimate of the likelihood of an extreme event
based on the statistical analysis of data (including flood records, fire
frequency, historical climatic records) that an event of a given magnitude will
occur in a given year. The probability can be used to assess the risk of these
events for human populations but can also be used by biologists when creating
habitat management plans or conservation plans for endangered species. The
concept is based on the magnitude-frequencyprinciple, where large magnitude
events (such as major hurricanes) are comparatively less frequent than smaller
magnitude incidents (such as rain showers). (For more information visit Climatica's Return Periods of Extreme Events.)

Question

Your friend is thinking about buying a house near Boulder Creek. The
house is above the level of seasonal high water but was flooded in the 2013
flood. He realizes how expensive flood insurance is and says, "Why do I have to
buy this insurance, a flood like that won't happen for another 100 years?
I won't live here any more." How would you explain to him that even though the
flood was a 100-year flood he should still buy the flood insurance?

Flood Plains

Definition: A flood plain is land adjacent to a waterway, from the
channel banks to the base of the enclosing valley walls, that experiences
flooding during periods of high discharge.

Flood plain through the city of Boulder. The LiDAR data used in
the lesson is from Four Mile Canyon Creek. Source:floodsafety.com.

1. Economic Impacts

2. Before & After Photos

We could view photos from before and after the disturbance event to see where
erosion or sedimentation has occurred.

Images are great for an overall impression of what happened, where soil has
eroded, and where soil or rocks have been deposited. But it is hard to
get measurements of change from these 2D images. There are several ways that we can
measure the apparent erosion and soil deposition.

3. Field Surveys

Standard surveys can be done to map the three-dimensional position of points allowing
for measurement of distance between points and elevation. However, this requires
extensive effort by land surveyors to visit each location and collect a large
number of points to precisely map the region. This method can be very time intensive.

Survey of a NEON field site done with a total station Source: Michael Patterson.

This method is challenging to use over a large spatial scale.

4. Stereoscopic Images

We could view stereoscopic images, two photos taken from slightly different
perspectives to give the illusion of 3D, one can view, and even measure,
elevation changes from 2D pictures.

A Sokkisha MS-16 stereoscope and the overlapping imaged used to
create 3-D visuals from a aerial photo. Source: Brian Haren.

However, this relies on specialized equipment and is challenging to automate.

5. LiDAR

A newer technology is Light Detection and Ranging (LiDAR or lidar).

Watch this video to see how LiDAR works.

Using LiDAR to Measure Change

LiDAR data allows us to create models of the earth's surface. The general term
for a model of the elevation of an area is a Digital Elevation Model (DEM). DEMs
come in two common types:

Digital Terrain Models (DTM): The elevation of the ground (terrain).

Digital Surface Models (DSM): The elevation of everything on the surface of the earth,
including trees, buildings, or other structures. Note: some DSMs have been post-processed to remove buildings and other human-made objects.

Digital Terrain Models (DTMs)

Here we have Digital Terrain Model of lower Four-Mile Canyon Creek from before
the 2013 flood event (left) and from after the 2013 flood event (right). We can
see some subtle differences in the elevation, especially along the stream bed,
however, even on this map it is still challenging to see.

Digital Elevation Model of Difference (DoD)

If we have a DEM from before and after an event, we can can create a model that
shows the change that occurred during the event. This new model is called a
Digital Elevation Model of Difference (DoD).

A cross-section showing the data represented by a Digital
Elevation Model of Difference (DoD) created by subtracting one DTM from
another. The resultant DoD shows the change that has occurred in a given
location- here, in orange, the areas where the earth's surface is lower than
before and, in teal, the areas where the earth's surface is higher than
before.

Questions

Here we are using DTMs to create our Digital Elevation Model of Difference (DoD)
to look at the changes after a flooding event. What types of disturbance events
or what research question might one want to look at DoDs from Digital Surface
Models?

Four Mile Canyon Creek DoD

Areas in red are those were the elevation is lower after the flood
and areas in blue are those where the elevation is higher after the flood event.

Using Data to Understand Disturbance Events

We've used the data from drought, atmospheric conditions, precipitation, stream flow,
and the digital elevation models to help us understand what caused the 2013
Boulder County, Colorado flooding event and where there was change in the stream
bed around Four Mile Canyon Creek at the outskirts of Boulder, Colorado.

Quantifying the change that we can see from images of the flooding streams or
post-flood changes to low lying areas allows scientists, city planners, and
homeowners to make choices about future land use plans.

Follow-up Questions

What other types of questions could this or similar data be used to answer?

What types of disturbance events in your local area could you use data to
quantify the causes and impact of?

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